Mask assembly and associated methods

ABSTRACT

A method comprising the steps of receiving a mask assembly comprising a mask and a removable EUV transparent pellicle held by a pellicle frame, removing the pellicle frame and EUV transparent pellicle from the mask, using an inspection tool to inspect the mask pattern on the mask, and subsequently attaching to the mask an EUV transparent pellicle held by a pellicle frame. The method may also comprise the following steps: after removing the pellicle frame and EUV transparent pellicle from the mask, attaching to the mask an alternative pellicle frame holding an alternative pellicle formed from a material which is substantially transparent to an inspection beam of the inspection tool; and after using an inspection tool to inspect the mask pattern on the mask, removing the alternative pellicle held by the alternative pellicle frame from the mask in order to attach to the mask the EUV transparent pellicle held by the pellicle frame.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/798,688, filed on Feb. 24, 2020, which is a divisional of U.S.application Ser. No. 15/545,390, 371(c) Date Jul. 21, 2017 (U.S. Pat.No. 10,571,800, Issued on Feb. 25, 2020), which is a National StageEntry of International Application No. PCT/EP2016/052055, InternationalFiling Date Feb. 1, 2016, which claims benefit of U.S. Application No.62/111,380, filed on Feb. 3, 2015 and of U.S. Application No.62/118,922, filed on Feb. 20, 2015 and of U.S. Application No.62/270,330, filed on Dec. 21, 2015, which are all incorporated herein inits entirety by reference.

FIELD

The present invention relates to a mask assembly, and particularly butnot exclusively to methods which use a mask assembly. A mask assemblymay comprise a mask and a pellicle. The present invention hasparticular, but not exclusive, use in connection with EUV lithographicapparatus and EUV lithographic tools.

BACKGROUND

A lithographic apparatus is a machine constructed to apply a desiredpattern onto a substrate. A lithographic apparatus can be used, forexample, in the manufacture of integrated circuits (ICs). A lithographicapparatus may for example project a pattern from a patterning device(e.g., a mask) onto a layer of radiation-sensitive material (resist)provided on a substrate.

The wavelength of radiation used by a lithographic apparatus to projecta pattern onto a substrate determines the minimum size of features thatcan be formed on that substrate. A lithographic apparatus that uses EUVradiation, being electromagnetic radiation having a wavelength withinthe range 4-20 nm, may be used to form smaller features on a substratethan a conventional lithographic apparatus (which may for example useelectromagnetic radiation with a wavelength of 193 nm).

A mask that is used to impart a pattern to a radiation beam in alithographic apparatus may form part of a mask assembly. A mask assemblymay include a pellicle that protects the mask from particlecontamination. The pellicle may be supported by a pellicle frame.

The use of pellicles in lithography is well-known and well-established.A typical pellicle in a DUV lithographic apparatus is a membrane whichis located away from the mask and is out of the focal plane of alithographic apparatus in use. Because the pellicle is out of the focalplane of the lithographic apparatus, contamination particles which landon the pellicle are out of focus in the lithographic apparatus.Consequently, images of the contamination particles are not projectedonto the substrate. If the pellicle were not present, then acontamination particle which landed on the mask would be projected ontothe substrate and would introduce a defect into the projected pattern.

It may be desirable to use a pellicle in an EUV lithographic apparatus.EUV lithography differs from DUV lithography in that it is typicallyperformed in a vacuum and the mask is typically reflective rather thanbeing transmissive. Challenges relating to the use of a pellicle for EUVlithography may arise which are not present when a pellicle for DUVlithography.

It may be desirable to provide a mask assembly and associated methodswhich overcome or mitigate a problem associated with the prior art.

SUMMARY

According to a first aspect of the invention there is provided a methodcomprising the steps of receiving a mask assembly comprising a mask anda removable EUV transparent pellicle held by a pellicle frame, removingthe pellicle frame and EUV transparent pellicle from the mask, using aninspection tool to inspect the mask pattern on the mask, andsubsequently attaching to the mask an EUV transparent pellicle held by apellicle frame.

The method is advantageous because it allows inspection of the maskwithout interference from the EUV transparent pellicle (which may beopaque to a beam used by the mask inspection tool).

The method may further comprise, after removing the pellicle frame andEUV transparent pellicle from the mask, attaching to the mask analternative pellicle frame holding an alternative pellicle formed from amaterial which is substantially transparent to an inspection beam of theinspection tool, and after using an inspection tool to inspect the maskpattern on the mask, removing the alternative pellicle held by thealternative pellicle frame from the mask in order to attach to the maskthe EUV transparent pellicle held by the pellicle frame.

Removing the pellicle frame from the mask may comprise disengaging anattachment mechanism from an attachment feature, and attaching thepellicle frame to the mask may comprise engaging the attachmentmechanism to an attachment feature. The attachment feature may becoupled to the mask and the attachment mechanism may be coupled to thepellicle frame. The attachment feature may be coupled to the mask alsoafter removing the pellicle frame and EUV transparent pellicle from themask by disengaging the attachment mechanism from the attachmentfeature, such that the attachment feature is available for subsequentattachment of an EUV transparent pellicle held by a pellicle frame afterinspecting the mask pattern on the mask. The alternative pellicle may beattached to the mask such that the attachment feature of the EUVtransparent pellicle does not touch the alternative pellicle.

The attachment mechanism may comprise a locking member configured toengage with an attachment feature comprising a protrusion.

The EUV transparent pellicle and pellicle frame which is subsequentlyattached to the mask may be the same EUV transparent pellicle andpellicle frame that was removed from the mask.

The alternative pellicle may be substantially transparent to a non-EUVradiation beam used by the mask inspection tool.

The non-EUV radiation beam used by the mask inspection tool may be a DUVradiation beam.

The alternative pellicle may be substantially transparent to a particlebeam used by the mask inspection tool.

The particle beam used by the mask inspection tool may be an electronbeam.

The alternative pellicle may be attached to the mask using an attachmentmechanism which is used solely for the alternative pellicle and is notused for the attachment of the EUV transparent pellicle.

The mask may be in a clean environment throughout the method.

The method may further comprise transferring the mask assembly inside asealed container from a lithographic apparatus to a pellicle removal andattachment tool.

The method may further comprise transferring one or more selected fromthe mask, the pellicle assembly or the mask assembly inside a sealedcontainer from a pellicle removal and attachment tool to a maskinspection tool.

The mask inspection tool may be integrated with the pellicle removal andattachment tool such that the mask assembly stays in the sameenvironment.

The method may further comprise cleaning the mask or the pellicle.

The sealed container may have a recessed portion configured toaccommodate sagging of the pellicle.

A separation between the recessed portion of the container and a planeof the pellicle of the mask assembly may be between is between 0.5 mmand 1 mm.

According to a second aspect of the invention there is provided a methodcomprising the steps of receiving a mask assembly comprising a mask andan EUV transparent pellicle held by a pellicle frame arranged to beremovably attachable to the mask, removing the pellicle frame and EUVtransparent pellicle from the mask, attaching to the mask an alternativepellicle held by an alternative pellicle frame arranged to be removablyattachable to the mask, wherein the alternative pellicle is formed froma material different to the material used to form the EUV transparentpellicle, which material is substantially transparent to an inspectionbeam of an inspection tool, using the inspection beam in the inspectiontool to inspect the mask pattern on the mask, removing the alternativepellicle from the mask, and subsequently attaching to the mask an EUVtransparent pellicle held by a pellicle frame.

The method is advantageous because it allows inspection of the maskwithout interference from the EUV transparent pellicle (which may beopaque to a beam used by the mask inspection tool).

The alternative pellicle frame may be attached to the mask at adifferent location than the EUV transparent pellicle frame.

According to a third aspect of the invention there is provided a maskassembly container comprising an opening through which a mask assemblymay be placed inside the container, and a seal which seals shut theopening when the mask assembly is located inside the container, whereinthe container has a floor configured to accommodate outward sagging ofthe pellicle.

Accommodating sagging of the pellicle in this manner is advantageousbecause it avoids the pellicle touching the container, which would beliable to damage the pellicle.

The floor may be between 0.5 mm and 1 mm or more away from a pellicleplane when the mask assembly is held in the sealed container.

According to a fourth aspect of the invention there is provided a maskprovided with a protrusion configured to receive a pellicle frameattachment mechanism, wherein a bottom surface of the protrusion has alip which defines a recess in the surface of the base, and wherein theprotrusion is attached to the mask by glue in the recess.

Attaching the protrusion in this manner is advantageous because itreduces the risk of unwanted outgassing from the glue.

The volume of the glue may be less than the volume of the recess.

The glue may pull the protrusion towards the mask such that the recessand the mask form a substantially enclosed space which retains the glue.

The protrusion may comprise an opening in the lip such that the recessand the mask form a space which is partially open for glue outgassing

The protrusion may be attached to substrate material of the mask.

According to a fifth aspect of the invention there is provided apellicle assembly container comprising an opening through which apellicle assembly may be placed inside the container, and a seal whichseals shut the opening when the pellicle assembly is located inside thecontainer, wherein the container has a floor configured to accommodateoutward sagging of the pellicle.

Accommodating sagging of the pellicle in this manner is advantageousbecause it avoids the pellicle touching the container, which would beliable to damage the pellicle.

According to a sixth aspect of the invention there is provided a maskprovided with at least three protrusions configured to receive apellicle frame attachment mechanism, wherein the protrusions areremovably attached to the mask.

Making the protrusions removably attachable is advantageous because itallows the mask to be cleaned in a straightforward manner without theprotrusions being present, following which the protrusions may bereattached to the mask.

The protrusions may be attached to substrate material of the mask.

According to a seventh aspect of the invention there is provided amethod of making a pellicle assembly, the method comprising forming amembrane on a substrate and etching away substrate material to exposethe membrane and thereby provide a pellicle membrane supported by asubstrate perimeter, attaching a support frame to a portion of thesubstrate which borders the membrane, providing a first cover on oneside of the substrate and a second cover on an opposite side of thesubstrate and clamping them together to form a sealed environment whichcontains the pellicle membrane.

The method is advantageous because the substrate provides support forthe membrane and preserves tautness of the membrane, whilst the coversact to protect the membrane.

The first cover may be clamped against the substrate.

The cover second cover may be clamped against the substrate.

The method may further comprise cutting away parts of the substratewhich project beyond the first and second covers.

The substrate may be a silicon wafer.

The second cover may cover the support frame such that the support frameis located within the sealed environment.

The first cover may include a recess configured to accommodate saggingof the pellicle membrane.

The method of making the pellicle assembly may be performed at apellicle manufacturing location.

According to an eighth aspect of the invention there is provided amethod comprising the above method of making the pellicle assembly, andfurther comprising forming a mask assembly by attaching a pelliclelocation tool to the support frame, removing the second cover from thepellicle assembly, attaching the support frame to a mask, and removingthe first cover from the pellicle assembly using the pellicle locationtool.

The pellicle location tool may include arms which are received in blindholes provided in the support frame.

The method of forming a mask assembly may be performed at a mask shop.

The method may further comprise putting the mask assembly inside acontainer and sealing that container.

According to a ninth aspect of the invention there is provided apellicle assembly comprising a pellicle membrane extending from asubstrate border portion, a support frame attached to the substrateborder portion, a first cover and a second cover, wherein the first andsecond covers are provided on opposite sides of the substrate borderportion and form a sealed environment which contains the pelliclemembrane.

The sealed environment is advantageous because it prevents contaminationfrom entering the environment and contaminating the pellicle membrane.

The second cover may cover the support frame such that the support frameis located within the sealed environment.

The first and second covers may be clamped against the substrate borderportion.

According to a tenth aspect of the invention there is provided a methodof monitoring a pellicle of a mask assembly, the mask assemblycomprising a pellicle assembly and a mask, the method comprisingmeasuring a property of the pellicle and monitoring for a change of theproperty which is associated with an increased risk of pelliclebreakage, and when such a change is seen removing the pellicle assemblyfrom the mask and replacing it with a new pellicle assembly.

The property of the pellicle may be measured when the mask assembly isin situ in the lithographic apparatus.

The property may be infra-red emission of the pellicle and/or may bedeflection of the pellicle during scanning movement of the maskassembly.

The method may comprise transferring the mask assembly to a maskassembly inspection tool and then measuring the property of the pellicleusing the mask assembly inspection tool.

One or more of the following measurement techniques may be used tomeasure one or more properties of the pellicle: EUV reflectionmeasurements, EUV transmission measurements, ellipsometry, Ramanspectroscopy, X-ray reflection measurements, microscope inspection,resonance measurements, scanning heat load measurements, pellicledeflection during pumpdown or venting.

The method may comprise removing the pellicle assembly from the mask,transferring the pellicle assembly to a pellicle assembly inspectiontool and then measuring the property of the pellicle using the pellicleassembly inspection tool.

One or more of the following measurement techniques may be used tomeasure one or more properties of the pellicle: EUV transmissionmeasurements (pellicle assembly removed from mask), EUV reflectionmeasurements, birefringence measurements, ellipsometry, Fouriertransform infra-red spectroscopy, Raman spectroscopy, X-ray reflectionmeasurements, microscope inspection, resonance measurements, measurementof pellicle displacement due to pressure difference, deflection duringpumpdown or/venting, scanning heat load measurements, frame deformationmeasurements.

Features of different aspects of the invention may be combined withfeatures of other aspects of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings, in which:

FIG. 1 is a schematic illustration of a lithographic system comprising alithographic apparatus and a radiation source;

FIG. 2 is a schematic illustration of a mask assembly according to anembodiment of the invention;

FIG. 3 is a schematic illustration of a protrusion which forms part ofthe mask assembly shown in FIG. 2 ;

FIG. 4 is a process flow which depicts a method according to anembodiment of the invention;

FIG. 5 is a schematic illustration of a mask assembly and containeraccording to an embodiment of the invention;

FIG. 6 is a process flow which depicts a method according to anembodiment of the invention;

FIG. 7 is a process flow which depicts a method according to anembodiment of the invention; and

FIG. 8 is a process flow which depicts a method according to anembodiment of the invention;

FIGS. 9A-9D schematically depict a method of making a pellicle assembly;and

FIGS. 10A-10C schematically depict a method of forming a mask assemblyusing the pellicle assembly; and

FIG. 11 is a process flow which depicts a method according to anembodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 shows a lithographic system including a mask assembly accordingto one embodiment of the invention. The lithographic system comprises aradiation source SO and a lithographic apparatus LA. The radiationsource SO is configured to generate an extreme ultraviolet (EUV)radiation beam B. The lithographic apparatus LA comprises anillumination system IL, a support structure MT configured to support amask assembly 15 including a mask MA, a projection system PS and asubstrate table WT configured to support a substrate W. The illuminationsystem IL is configured to condition the radiation beam B before it isincident upon the mask MA. The projection system is configured toproject the radiation beam B (now patterned by the mask MA) onto thesubstrate W. The substrate W may include previously formed patterns.Where this is the case, the lithographic apparatus aligns the patternedradiation beam B with a pattern previously formed on the substrate W.

The radiation source SO, illumination system IL, and projection systemPS may all be constructed and arranged such that they can be isolatedfrom the external environment. A gas at a pressure below atmosphericpressure (e.g., hydrogen) may be provided in the radiation source SO. Avacuum may be provided in the illumination system IL and/or theprojection system PS. A small amount of gas (e.g., hydrogen) at apressure well below atmospheric pressure may be provided in theillumination system IL and/or the projection system PS.

The radiation source SO shown in FIG. 1 is of a type that may bereferred to as a laser produced plasma (LPP) source. A laser 1, whichmay for example be a CO2 laser, is arranged to deposit energy via alaser beam 2 into a fuel, such as tin (Sn) that is provided from a fuelemitter 3. Although tin is referred to in the following description, anysuitable fuel may be used. The fuel may for example be in liquid form,and may for example be a metal or alloy. The fuel emitter 3 may comprisea nozzle configured to direct tin, e.g., in the form of droplets, alonga trajectory towards a plasma formation region 4. The laser beam 2 isincident upon the tin at the plasma formation region 4. The depositionof laser energy into the tin creates a plasma 7 at the plasma formationregion 4. Radiation, including EUV radiation, is emitted from the plasma7 during de-excitation and recombination of ions of the plasma.

The EUV radiation is collected and focused by a near normal incidenceradiation collector 5 (sometimes referred to more generally as a normalincidence radiation collector). The collector 5 may have a multilayerstructure that is arranged to reflect EUV radiation (e.g., EUV radiationhaving a desired wavelength such as 13.5 nm). The collector 5 may havean elliptical configuration, having two ellipse focal points. A firstfocal point may be at the plasma formation region 4, and a second focalpoint may be at an intermediate focus 6, as discussed below.

In other embodiments of a laser produced plasma (LPP) source thecollector 5 may be a so-called grazing incidence collector that isconfigured to receive EUV radiation at grazing incidence angles andfocus the EUV radiation at an intermediate focus. A grazing incidencecollector may, for example, be a nested collector, comprising aplurality of grazing incidence reflectors. The grazing incidencereflectors may be disposed axially symmetrically around an optical axisO.

The radiation source SO may include one or more contamination traps (notshown). For example, a contamination trap may be located between theplasma formation region 4 and the radiation collector 5. Thecontamination trap may for example be a rotating foil trap, or may beany other suitable form of contamination trap.

The laser 1 may be separated from the radiation source SO. Where this isthe case, the laser beam 2 may be passed from the laser 1 to theradiation source SO with the aid of a beam delivery system (not shown)comprising, for example, suitable directing mirrors and/or a beamexpander, and/or other optics. The laser 1 and the radiation source SOmay together be considered to be a radiation system.

Radiation that is reflected by the collector 5 forms a radiation beam B.The radiation beam B is focused at point 6 to form an image of theplasma formation region 4, which acts as a virtual radiation source forthe illumination system IL. The point 6 at which the radiation beam B isfocused may be referred to as the intermediate focus. The radiationsource SO is arranged such that the intermediate focus 6 is located ator near to an opening 8 in an enclosing structure 9 of the radiationsource.

The radiation beam B passes from the radiation source SO into theillumination system IL, which is configured to condition the radiationbeam. The illumination system IL may include a facetted field mirrordevice 10 and a facetted pupil mirror device 11. The faceted fieldmirror device 10 and faceted pupil mirror device 11 together provide theradiation beam B with a desired cross-sectional shape and a desiredangular distribution. The radiation beam B passes from the illuminationsystem IL and is incident upon the mask assembly 15 held by the supportstructure MT. The mask assembly 15 includes a mask MA and a pellicle 19,which is held in place by a pellicle frame 17. The mask MA reflects andpatterns the radiation beam B. The illumination system IL may includeother mirrors or devices in addition to or instead of the faceted fieldmirror device 10 and faceted pupil mirror device 11.

Following reflection from the mask MA the patterned radiation beam Benters the projection system PS. The projection system comprises aplurality of mirrors that are configured to project the radiation beam Bonto a substrate W held by the substrate table WT. The projection systemPS may apply a reduction factor to the radiation beam, forming an imagewith features that are smaller than corresponding features on the maskMA. A reduction factor of 4 may for example be applied. Although theprojection system PS has two mirrors in FIG. 1 , the projection systemmay include any number of mirrors (e.g., six mirrors).

The lithographic apparatus may, for example, be used in a scan mode,wherein the support structure (e.g., mask table) MT and the substratetable WT are scanned synchronously while a pattern imparted to theradiation beam is projected onto a substrate W (i.e., a dynamicexposure). The velocity and direction of the substrate table WT relativeto the support structure (e.g., mask table) MT may be determined by thedemagnification and image reversal characteristics of the projectionsystem PS. The patterned radiation beam that is incident upon thesubstrate W may comprise a band of radiation. The band of radiation maybe referred to as an exposure slit. During a scanning exposure, themovement of the substrate table WT and the support structure MT may besuch that the exposure slit travels over an exposure field of thesubstrate W.

The radiation source SO and/or the lithographic apparatus that is shownin FIG. 1 may include components that are not illustrated. For example,a spectral filter may be provided in the radiation source SO. Thespectral filter may be substantially transmissive for EUV radiation butsubstantially blocking for other wavelengths of radiation such asinfra-red radiation.

In other embodiments of a lithographic system the radiation source SOmay take other forms. For example, in alternative embodiments theradiation source SO may comprise one or more free electron lasers. Theone or more free electron lasers may be configured to emit EUV radiationthat may be provided to one or more lithographic apparatus.

As was described briefly above, the mask assembly 15 includes a pellicle19 that is provided adjacent to the mask MA. The pellicle 19 is providedin the path of the radiation beam B such that radiation beam B passesthrough the pellicle 19 both as it approaches the mask MA from theillumination system IL and as it is reflected by the mask MA towards theprojection system PS. The pellicle 19 comprises a thin film that issubstantially transparent to EUV radiation (although it will absorb asmall amount of EUV radiation). The pellicle 19 acts to protect the maskMA from particle contamination. The pellicle 19 may be herein referredto as an EUV transparent pellicle.

Whilst efforts may be made to maintain a clean environment inside thelithographic apparatus LA, particles may still be present inside thelithographic apparatus LA. In the absence of a pellicle 19, particlesmay be deposited onto the mask MA. Particles on the mask MA maydisadvantageously affect the pattern that is imparted to the radiationbeam B and the pattern that is transferred to the substrate W. Thepellicle 19 advantageously provides a barrier between the mask MA andthe environment in the lithographic apparatus LA in order to preventparticles from being deposited on the mask MA.

The pellicle 19 is positioned at a distance from the mask MA that issufficient that any particles that are incident upon the surface of thepellicle 19 are not in the focal plane of the radiation beam B. Thisseparation between the pellicle 19 and the mask MA, acts to reduce theextent to which any particles on the surface of the pellicle 19 impart apattern to the radiation beam B. It will be appreciated that where aparticle is present in the beam of radiation B, but at a position thatis not in a focal plane of the beam of radiation B (i.e., not at thesurface of the mask MA), then any image of the particle will not be infocus at the surface of the substrate W. In some embodiments, theseparation between the pellicle 19 and the mask MA may, for example, bebetween 2 mm and 3 mm (e.g. around 2.5 mm).

FIG. 2 depicts the mask assembly 15 in cross-section and in more detail.The mask MA has a patterned surface 24. The pellicle frame 17 whichsupports the pellicle 19 is provided with an attachment mechanism 22.The attachment mechanism 22 may be configured to allow the pellicleframe to be removably attachable to the mask MA (i.e. to allow thepellicle frame to be attachable to and detachable from the mask). Theattachment mechanism 22 is configured to engage with an attachmentfeature (not shown) provided on the mask MA. The attachment feature may,for example, be a protrusion which extends from the mask MA. Theattachment mechanism 22 may, for example, comprise a locking memberwhich engages with the protrusion and secures the pellicle frame 17 tothe mask MA.

A plurality of attachment mechanisms and associated attachment featuresmay be provided. The attachment mechanisms may be distributed around thepellicle frame 17 (e.g. two on one side of the frame and two on anopposite side of the frame). Associated attachment features may bedistributed around the perimeter of the mask MA.

The attachment mechanism 22 may suspend the pellicle frame 17 relativeto the mask MA. That is, there may be a separation between the pellicleframe 17 and the mask MA, such that a gap exists between them throughwhich gas may flow into and out of the space between the pellicle 19 andthe mask. The gap may be in the form of a slit which extends round theperimeter of the pellicle frame 17, the slit being interrupted byattachment mechanisms which connect the pellicle frame to the mask MA.The separation between the pellicle frame 17 and the mask MA may forexample be between 200 microns and 300 microns. Since this separation isrelatively narrow, the flow of gas into and out of the space between thepellicle 19 and the mask MA is restricted.

In an alternative embodiment, the attachment mechanism may be such thatthe pellicle frame 17 is in contact with the mask MA.

As noted above, a plurality of attachment mechanisms and associatedattachment features may be provided. Each attachment mechanism may forexample comprise a locking member and each associated attachment featuremay comprise a protrusion (which may be referred to as a stud). Thelocking members and protrusions may form kinematic connections betweenthe pellicle frame and the mask. This may allow the pellicle frame to bemounted on the mask without causing significant distortion of the mask.

The attachment mechanism may be configured to avoid lateral slidingmotion between the attachment mechanism and the attachment feature whenattaching the mask frame to the mask, including also no sliding motionat the subsequent pellicle attachment after mask inspection hasoccurred. Avoiding such sliding motion provides the advantage thatcontamination particles which might otherwise be generated are avoided.

In an embodiment, depicted in FIG. 3 , the protrusion 27 extends fromthe mask MA and includes a distal head 28. The locking member may beconfigured to engage the protrusion 27 (stud) beneath the distal head 28and thereby secure the pellicle frame to the mask. The attachmentmechanism may include a resilient feature which is configured to biasthe locking member against the stud. The locking member may bedisengaged from the stud by applying force against the resilient bias.

Although FIG. 3 depicts a protrusion 27 extending from the face of themask MA, in an embodiment protrusions may instead be provided on sidesof the mask. In an embodiment, some protrusions may be provided on sidesof the mask and some protrusions may be provided on the face of themask.

As may be seen from FIG. 3 , in an embodiment the protrusion includes abase 29 which is fixed to the mask MA. A lip 31 is provided on a bottomsurface of the base, the lip defining a recess 33 in the surface of thebase. Glue is provided in the recess 33 to secure the base 29 to themask MA. The volume of glue provided in the recess is less than thevolume of the recess, and when the base is pressed against the mask MAthe glue remains fully within the recess. The lip 31 is held against themask MA by the glue because the glue shrinks as it dries and therebypulls the base 29 towards the mask MA. Consequently, the recess and themask MA together define a substantially enclosed space which retains theglue. This is advantageous because outgassing of glue into a cleanenvironment is substantially avoided.

Gluing of the protrusion to the mask MA may however be achieved in anyother suitable manner. In another embodiment a groove, for example asubstantially U-shaped groove, may be provided for venting in the baseof the protrusion. In such case the U-shaped groove defines (togetherwith the mask MA) an island in the base surface of the protrusion. Theglue will be drawn in to the groove by capillary action and will securethe protrusion to the mask MA while the groove is still partially openfor some glue outgassing.

In yet another embodiment wherein the protrusion has lip 31 definingrecess 33, for venting it is sufficient to have one or more openings inthe lip 31 such that recess 33 is partially open for some glueoutgassing.

In the above embodiments the groove in the base of the protrusion or theopening in the protrusion lip 31 is arranged such that outgassing of theglue is prevented from directly entering the space between the pellicleand the mask. Therefore, although some outgassing of the glue may occurover time, the outgassing will occur towards the outer side of thepellicle frame and thus will not cause significant contamination of thepellicle or the patterned mask area which may be sensitive tocontamination, thereby preventing haze on the mask.

The volume of glue used may for example be around 50 microns. The gluemay for example be Araldite® type from Huntsman Advanced Materials, anepoxy adhesive such as Epotec® or any other EUV suitable soft or hardglue.

In an embodiment in which the protrusion 27 extends from the face of themask MA, the protrusion may be located a few millimeters away from thepatterned surface of the mask. In such an embodiment, material whichoutgasses from the base 29 of the protrusion 27 may be incident upon thepatterned surface of the mask MA and may cause defects in projectedpatterns. In such circumstances, avoiding outgassing of the glue whichsecures the protrusion 27 to the mask MA is particularly advantageous.Providing glue in a substantially enclosed space which retains the glue(as described above) provides this advantage.

The bonding between the protrusion 27 and the mask may be permanent ortemporary. The protrusion may be fixed to the mask MA for example withan adhesive (e.g. as described above), or by using other means. Forexample, the protrusion may be fixed to the mask using a mechanicalattachment such as screws or clamps, via electrostatic or magneticforces induced to attract the protrusions to the mask, via opticalbonding (using the van der Waals adhesion force) or any other suitablemeans. Preferably the attachment of the protrusion to the mask is donein a manner which allows the protrusion to be is easily and cleanlyremovable (e.g. so that substantially no particles or outgassingmolecules and species are released which could deposit on the mask MA).In an alternative embodiment, the protrusions may be integral part ofthe mask MA (i.e. non-removable).

A multilayer stack of material may be provided across the mask MA toprovide EUV reflectivity. This multilayer stack may be partially coveredwith an EUV absorbing layer, this partial covering providing the maskwith a pattern to be projected onto substrates by the lithographicapparatus. An outer border of the mask may be provided with otherpatterns which are not projected onto substrates but instead have otheruses. For example, these patterns may include alignment marks and mayindicate the identity of the mask.

In an embodiment (which may be combined with any other embodimentdescribed in this document), a portion of the surface of the mask MA towhich the protrusion 27 is attached does not include a multilayer stackof material or an EUV absorbing layer. Instead these are removed fromthat portion of the surface of the mask MA (or were never present atthat portion of the surface of the mask). Consequently, the protrusionis attached directly to the material from which the mask is formed. Thismay be referred to as attaching the protrusion to substrate material ofthe mask (or equivalently referred to as attaching the protrusion to themask substrate). The mask substrate may for example be formed fromglass. The mask substrate may for example be formed from Low ThermalExpansion Material (LTEM).

Attaching the protrusion in this way is advantageous because thestrength of the connection between the protrusion 27 and the mask MA isnot influenced by material properties of the multilayer stack or theabsorbing layer. Instead the connection is determined only by thematerial of the mask substrate. A further advantage is that because theprotrusion 27 is not attached to the multilayer stack or the absorbinglayer, damage to those layers and consequent generation of contaminationparticles when attaching the protrusion is avoided.

An additional advantage is that it is easier to remove the protrusion 27and glue from the mask MA at a later time if this becomes necessary. Inparticular, any glue removal process does not risk damaging themultilayer stack or the absorbing layer because these are not presentwhere the glue is located. Again, this avoids consequent generation ofcontamination particles.

In an alternative arrangement the attachment mechanism(s) may beprovided on the mask and the attachment features(s) may be provided onthe pellicle frame.

A contamination particle 26 is schematically shown in FIG. 2 . Thecontamination particle 26 was incident upon the pellicle 19 and is heldby the pellicle. The pellicle 19 holds the contamination particlesufficiently far from the patterned surface 24 of the mask MA that it isnot imaged onto substrates by the lithographic apparatus LA.

The pellicle 19 may, for example, be formed from a material such aspolysilicon (pSi) film. Polysilicon (pSi) film is substantiallytransparent to EUV radiation. The pellicle 19 may alternatively beformed from some other material which is substantially transparent toEUV radiation, for example graphene, silicene, etc. By EUV transparentpellicle or a film substantially transparent for EUV radiation herein ismeant that the pellicle 19 transmits at least 65% of incident EUVradiation, preferably at least 80% and more preferably at least 90% ofincident EUV radiation. A capping layer which may help to reduce theeffect of hydrogen radicals, plasma and traces of oxygen on the pellicle19 may be provided. The capping layer may be provided both on thepellicle and on the pellicle frame.

A mask assembly according to an embodiment of the invention may providea mask pattern which remains substantially defect free during use (themask pattern is protected from contamination by the pellicle). As notedabove, a separation may be provided between the pellicle frame and themask (e.g. in the form of slits) which allow some gas to flow into andout of the space between the pellicle and the mask. This allows pumpingdown and venting of the mask assembly to be performed without damagingthe mask assembly.

FIG. 4 is a process flow which illustrates handling of a mask assemblyto allow inspection of the mask pattern by a mask inspection tool.Although the pellicle 19 is substantially transparent to EUV radiation,it may be substantially opaque to an inspection beam used by the maskinspection tool (or at least not sufficiently transparent to allow themask inspection tool to inspect the mask pattern correctly). The processflow addresses this problem. The mask inspection tool may for exampleuse as an inspection beam a radiation beam at a non-EUV wavelength (e.g.DUV, VIS or IR radiation). The mask inspection tool may for example useas an inspection beam a particle beam such as an electron beam (e-beam).

The mask assembly is transferred from a lithographic apparatus to apellicle removal and attachment tool. Inside the pellicle removal andattachment tool a controlled clean environment is provided. The pellicleremoval and attachment tool includes attachment mechanism actuatorswhich are configured to disengage the attachment mechanisms 22 (see FIG.2 ) from the mask MA. These actuators are used to disengage theattachment mechanisms 22, and the pellicle frame 17 (with the EUVtransparent pellicle 19) is then removed from the mask MA.

An alternative pellicle is then attached to the mask MA. The alternativepellicle is formed from a different material to the EUV transparentpellicle. The alternative pellicle may be formed from a material such asan amorphous fluoropolymer (e.g. Teflon AF or Cytop) and issubstantially transparent to an inspection beam used by the maskinspection tool (e.g. a DUV radiation beam or e-beam). The term“substantially transparent to an inspection beam” is intended to meanthat the alternative pellicle transmits the inspection beam sufficientlywell to allow an inspection of the mask to be performed. The alternativepellicle may for example transmit at least 80% of the inspection beam,more preferably at least 90% of the inspection beam.

The alternative pellicle may be attached to the same attachment featuresas used to attach the EUV transparent pellicle to the mask MA. Inanother arrangement, the alternative pellicle may be attached to themask MA using alternative attachment features which are used solely forreceiving the alternative pellicle and is not used for the EUVtransparent pellicle. The alternative attachment features may beprovided further away from the patterned area of the mask than theattachment features used by the EUV transparent pellicle. Usingalternative attachment features is advantageous because it avoids thepossibility of the EUV transparent pellicle attachment features beingdamaged when attaching the alternative pellicle (the EUV transparentpellicle attachment features are untouched when the attaching thealternative pellicle). Providing the alternative attachment featuresfurther away from the patterned area of the mask is advantageous becauseit reduces the risk of contamination particles travelling from theattachment features to the patterned area. The alternative attachmentfeatures may for example be provided on sides of the mask MA.

The alternative pellicle may be provided with attachment mechanismsconfigured to engage with the alternative attachment features providedon the mask MA. The alternative attachment mechanisms may for example beprovided on a frame which supports the alternative pellicle.

Since the alternative pellicle is in place during inspection of the maskby a mask inspection tool, the alternative pellicle may be referred toas an inspection-compatible pellicle (since it is substantiallytransparent to the inspection beam). The alternative pellicle may alsobe referred to as a temporary pellicle.

The mask assembly, which now comprises the mask together with thealternative pellicle, is then transferred to the mask inspection tool.The mask inspection tool checks for contamination on the mask pattern.If contamination is found, then cleaning of the mask MA may be performedto remove the contamination. The alternative pellicle may be removed inorder to allow the mask to be cleaned (e.g. using the pellicle removaland attachment tool). The attachment features (e.g. protrusions) mayremain in place on the mask MA during this cleaning of the mask. Thisincludes the attachment features used to receive the EUV transparentpellicle, and may also include attachment features used to receive thealternative pellicle (if such attachment features are present).Following cleaning, the alternative pellicle may be reattached to themask MA (e.g. using the pellicle removal and attachment tool). The maskassembly is returned to the mask inspection tool where a furtherinspection is performed to check that the contamination has beenremoved.

The mask assembly is then transferred from the mask inspection tool tothe pellicle removal and attachment tool. The pellicle removal andattachment tool then removes the alternative pellicle from the mask MA.

A pellicle frame and EUV transparent pellicle is then attached to themask MA. This may be the same pellicle frame and pellicle that waspreviously removed from the mask MA, or may be a new pellicle frame andpellicle. If the same pellicle frame and pellicle is reused, then thepellicle may be cleaned before being reattached to the mask MA.

The mask assembly 15, which now comprises a mask MA, pellicle frame 17and EUV transparent pellicle 19 is then transferred to a lithographicapparatus. The lithographic apparatus is used to project patterns fromthe mask MA onto substrates.

The mask inspection tool and the pellicle removal and attachment toolmay be integrated with each other in order to minimize transportation ofthe mask assembly.

The alternative pellicle may be a DUV-transparent pellicle, or anysuitable pellicle (i.e. inspection-compatible or a temporary pellicle)for use in place of the EUV transparent pellicle. The alternativepellicle may be substantially transparent to a radiation beam orparticle beam (e.g. e-beam) used by a mask inspection tool.

In an alternative approach the pellicle frame 17 and EUV transparentpellicle 19 may be removed from the mask MA using the pellicle removaland attachment tool and then passed to the mask inspection tool withoutattaching an alternative pellicle to the mask. Following inspection ofthe mask MA by the mask inspection tool the pellicle frame 17 and EUVtransparent pellicle 19 may be re-attached to the mask by the pellicleremoval and attachment tool (or a new pellicle frame 17 and EUVtransparent pellicle 19 may be attached). Although this approach allowsinspection of the mask it includes the disadvantage that the mask is notprotected by a pellicle during inspection of the mask, or duringtransfer to and from the mask inspection tool. The mask inspection toolmay for example have a less closely controlled clean environment thanthe environment of the pellicle removal and attachment tool or theenvironment of a lithographic apparatus. A contamination particle couldfor example adhere to the mask MA after inspection and before thepellicle frame 17 and EUV transparent pellicle 19 are attached to themask. Since this occurred after mask inspection the contaminationparticle would not be detected and would lead to defects in patternsprojected on substrates. This disadvantage is avoided by the methodshown in FIG. 4 because the mask MA is protected by a pellicle duringmask inspection and during transfer to and from the mask inspectiontool. The mask MA is only unprotected during swapping between the EUVtransparent pellicle and the alternative (e.g. DUV-transparent)pellicle, which is a small part of the process. The environment providedin the pellicle removal and attachment tool may be closely controlled(e.g. more closely controlled than other environments) given that thisis the only environment in which mask MA is unprotected.

In an embodiment, the mask MA may be fitted with an alternative pelliclerather than an EUV transparent pellicle during periods of non-use. Thealternative pellicle may for example comprise a DUV-transparentpellicle. DUV-transparent pellicle materials may suffer less fromoutgassing than EUV transparent pellicle materials, and thus lesscontamination due to outgassing over time may be expected when storageof a mask assembly with a DUV-transparent pellicle is used.

Although described above in the context of removing a pellicle framefrom a mask and subsequently re-attaching the pellicle frame, thepellicle removal and attachment tool may also be used to attach apellicle frame to a mask which has not previously been provided with apellicle.

During the process depicted in FIG. 4 the mask assembly is at all timesheld in a clean environment. The interior of the pellicle removal andattachment tool is a controlled clean environment, as is the interior ofthe mask inspection tool. Transfer of the mask assembly between thepellicle removal and attachment tool and the mask inspection tool may beachieved in one of two ways. The pellicle removal and attachment toolmay be connected to the mask inspection tool by a port which directlyconnects them together. The port is sealed from the exteriorenvironment. The port may connect the controlled clean environments ofthese two tools together, thereby allowing the mask assembly to traveldirectly from the pellicle removal and attachment tool to the maskinspection tool without leaving the controlled clean environment.

In an alternative approach the mask assembly may be placed in acontainer which provides a clean environment, and may then betransferred from the pellicle removal and attachment tool to the maskinspection tool inside the container. The interior of the container maybe connected to the controlled clean environment of the pellicle removaland attachment tool, such that the mask assembly can be placed into thecontainer (e.g. using a suitable handler) without leaving the controlledclean environment. The container is then sealed such that a cleanenvironment is maintained within the container. The container is thentaken to the mask inspection tool. At the mask inspection tool the cleanenvironment within the container is connected to the controlled cleanenvironment within the mask inspection tool, and the mask assembly isthen transferred into the mask inspection tool. The above steps arereversed to transfer the mask assembly back to the pellicle removal andattachment tool.

The container may also be used to provide a clean environment fortransfer of a pellicle assembly comprising a pellicle and pellicleframe. For example, a pellicle assembly may be transported to thepellicle removal and attachment tool using the container.

In an embodiment there is provided a container with an exchangeable viewwindow which is arranged to be assembled with the mask into a maskassembly (or into a pellicle assembly) in the lithographic apparatus. Byproviding an exchangeable film (an exchangeable view window) to thecontainer which will be transferred to the scanner or inspection tools,the mask will be constantly protected. The main exposure to particlestakes place in such case in a controlled environment, when the reticleis exchanged between containers. The protective exchangeable film may bechosen to be transparent for EUV during exposure time, and exchanged tobe transparent for inspection when provided to the inspection apparatus.The bottom side of the container, in vicinity of the exchangeable film,can be closed to protect the fragile film. During inspection the maskwith protective film is loaded to a reticle stage and the exchangeablefilm is chosen to be compatible with the light source used duringinspection. Then the mask is changed from a container A (with filmsuitable for inspection) to container B (with film suitable forexposure) in a clean environment to minimize particles. During exposurethe mask with protective film is loaded to the reticle stage and theexchangeable film is chosen to be compatible with light source usedduring exposure.

An example of a container which may be used to transfer a mask assembly(or a pellicle assembly) between controlled clean environments is shownschematically in FIG. 5 . The container 30 has a shape which generallycorresponds with the shape of the mask assembly 15. The container 30includes a plate 32 which includes a recessed portion 33. The recessedportion 33 is spaced apart from the pellicle 19 to accommodate somesagging of the pellicle. The separation between the recessed portion 33and the plane of the pellicle may for example be between around 0.5 mmand around 1 mm (e.g. around 0.7 mm or more). The term ‘plane of thepellicle’ may be interpreted as referring to a plane which correspondswith edges of the pellicle and in which the pellicle would lie if it didnot suffer from sagging.

Outward sagging of the pellicle 19 may occur if the pressure in thespace between the pellicle 19 and the mask MA is greater than thepressure outside of that space. This may occur if the pressure in thecontrolled clean environment of the container 30 is being reduced,because although there may be slits between the pellicle frame 17 andthe mask MA these slits may be relatively small and may restrict theflow of gas. Outward sagging of the pellicle 19 may also occur due togravity.

The container 30 may correspond generally with known containersconfigured to transport an EUV mask without a pellicle, except that theknown containers do not include a recess 33. The depth of the recess 33may for example be around 3 mm. This accommodates the pellicle frame 17and pellicle 19, which as noted above may have a height between around 2mm and around 2.5 mm, and provides space to accommodate outward saggingof the pellicle.

The container 30 further comprises a cover 34 which may be placed overthe mask assembly 15. The cover has the form of an open box (i.e. a boxwith no lid). The plate 32 acts as a floor which comes together with thecover 34 to form the container 30 which encloses the mask assembly 15. Aseal 36 is provided between the plate 32 and the cover 34, the sealacting to isolate the interior of the container from the externalenvironment. The seal 36 may have any suitable form (the depicted blackdiscs are merely illustrative).

Any suitable opening form of opening may be used to allow the maskassembly 15 to be placed inside the container 30.

The container 30 may be used to transport the mask assembly 15 (or apellicle assembly) between locations which are at vacuum and locationswhich are not at vacuum. The vacuum and non-vacuum locations may all becontrolled clean environments. For example, the pellicle removal andattachment tool may be not at vacuum, and the mask inspection tool maybe not at vacuum. Where this is the case, the container may take themask assembly 15 (or the pellicle assembly) from a vacuum environment(e.g. inside a lithographic apparatus) to a non-vacuum environment andvice versa.

The container 30 may include a port (not shown) through which gas can beintroduced to bring the interior of the container from a vacuum up toatmospheric pressure. As noted further above, although there may beslits between the pellicle frame 17 and the mask MA, these may berelatively small and may restrict the flow of gas into the space betweenthe pellicle 19 and the mask MA. When gas is being introduced into thecontainer 30 sagging of the pellicle 19 towards the mask MA may takeplace. However, the separation between the plane of the pellicle 19 andthe mask MA is arranged to be sufficiently large that the pellicle willnot come into contact with the mask. The flow of gas into the container30 may be controlled to ensure that the difference between pressures oneither side of the of the pellicle 19 remains below a desired thresholdlevel (which may be sufficiently low that it avoid the possibility ofthe pellicle touching the mask MA and also avoids breaking thepellicle).

Gas may be pumped out of the port as desired in order to provide avacuum within the container 30 prior to transferring the mask assembly15 to a lithographic apparatus. The gas may be pumped out at a ratewhich is sufficiently low that the difference between pressures oneither side of the of the pellicle 19 remains below a desired thresholdlevel (the threshold may be sufficiently low that it avoid thepossibility of the pellicle touching the container 30 and avoidsbreaking the pellicle).

In an embodiment in which the pellicle assembly is being transportedwithout the mask MA, pressure build-up on one side of the pellicle maynot happen. Nevertheless, sagging of the pellicle 19 may still occur,for example due to gravity. The recess 33 of the container 30 may have adepth which is larger than the extent to which the pellicle will sag.This prevents touching of the pellicle against the container andpotential resulting damage to the pellicle. The depth of the recess 33may for example be around 3 mm.

FIG. 6 is a process flow which illustrates at high level steps of amethod according to an embodiment of the invention. As depicted in FIG.6 , some of those steps may be performed at a pellicle manufacturingsite, some steps may be performed at a mask manufacturing site (whichmay be referred to as a mask shop), and some steps may be performed at alithographic FAB (where integrated circuits may be fabricated).

At the pellicle manufacturing site a pellicle is formed from a suitablematerial such as polysilicon and is bonded (e.g. glued) to a pellicleframe. The pellicle and pellicle frame are then inspected forcontamination. If contamination is found, then the pellicle and pellicleframe are cleaned to remove that contamination. The pellicle andpellicle frame are then transported in a container which provides aclean environment to the mask shop. The container may, for example,correspond with the container described above in connection with FIG. 5.

At the mask shop a mask is manufactured. This includes providing apattern on the mask which is subsequently to be projected ontosubstrates by a lithographic apparatus. The pellicle frame is secured tothe mask to form a mask assembly which comprises the pellicle, pellicleframe and mask. The mask assembly is then inspected for contamination.If contamination is found, then the mask assembly is cleaned to removethe contamination (for example the pellicle is removed, the mask iscleaned and the same or a new pellicle is reattached). The mask assemblyis then placed in a container which provides a clean controlledenvironment and is transported to a lithographic FAB. The container may,for example, correspond with the container described above in connectionwith FIG. 5 .

At the lithographic FAB the mask assembly is transferred from thecontainer into a lithographic apparatus. The lithographic apparatusprojects the pattern from the mask onto substrates in a conventionalmanner. The mask assembly is periodically inspected for pelliclecontamination and/or mask contamination. Inspection for pelliclecontamination may, for example, take place within the lithographicapparatus (however it also may be done in an independent tool outside ofthe lithographic apparatus). Inspection of the mask pattern may, forexample, be performed using a mask inspection tool. The mask assemblymay be cleaned as necessary and then used again to project patterns ontosubstrates.

FIG. 7 shows in more detail the mask assembly manufacturing steps whichmay be performed at the mask shop. A mask is manufactured in aconventional manner. The mask is a reflective mask for use in an EUVlithographic apparatus. Following manufacture the mask is cleaned inorder to remove contamination which may have been generated duringmanufacture of the mask. The back surface of the mask is then inspectedfor contamination (a contamination particle on the back surface of themask may cause unwanted localized distortion of the mask during use).The patterned surface of the mask is then inspected for contamination(as noted above such contamination may introduce defects into aprojected pattern). In an embodiment, the order of the inspection may bereversed, i.e. first inspect the patterned surface and then the backsurface. In an embodiment one of the inspections may be skipped (e.g.the back surface is not inspected).

A pellicle mounted on a pellicle frame is received from a pelliclemanufacturer. The pellicle is attached to the mask to form a maskassembly. Attaching the pellicle frame to the mask may comprise engagingan attachment mechanism to an attachment feature (although any otherbonding/attachment form is also possible). The attachment mechanism maycomprise a locking member configured to engage with a protrusion. In anembodiment, a plurality of projections (e.g. studs) may extend from themask. The pellicle frame may be provided with locking members whichengage with the protrusions and secure the pellicle frame to the mask.The protrusions may be provided on the front and/or on sides of themask.

The pellicle of the mask assembly is inspected for contamination.Attaching/removing the pellicle frame to the mask and inspecting thepellicle for contamination may be performed by the same tool. Althoughmentioned here in relation to the mask shop this may also be the case inthe lithographic FAB.

The pellicle may be removed and cleaned if contamination is found.Inspection of the mask pattern may be required. If this is not required,then the mask assembly is placed in a container for transportation tothe lithographic FAB. The container may, for example, correspond withthe container described above in connection with FIG. 5 .

If inspection of the mask pattern is required, then this may beperformed using a method which corresponds with that described above inconnection with FIG. 4 . That is, the pellicle and pellicle frame (whichare EUV transparent) may be removed and replaced with an alternativepellicle. This will allow inspection of the mask pattern using a maskinspection tool (as explained further above). The inspection may beperformed without removing the protrusions from the mask. Followinginspection of the mask pattern, the alternative pellicle is replacedwith the EUV transparent pellicle. The resulting mask assembly may thenbe transported to the lithographic FAB in a container.

As noted in FIG. 7 , and as described further above, inspection of themask pattern may be performed with no pellicle being present (i.e.neither the (EUV transparent pellicle nor an alternative pellicle arepresent). A disadvantage of this approach is that contamination may beintroduced onto the mask pattern whilst the pellicle is not present. Theinspection may be performed without removing the protrusions from themask.

If contamination is found then the mask is cleaned to remove thecontamination. Cleaning of the mask may be performed without a pelliclebeing present. The protrusions may remain on the mask during cleaning ofthe mask. The protrusions may be permanently bonded to the mask (i.e.the protrusions are a non-removable part of the mask).

In some cases it may be desirable to remove the protrusions from themask as part of the mask cleaning process. If the protrusions have beenattached to the mask by glue then the process used to remove theprotrusions may depend upon the form of glue that was used to attach theprotrusions. If the glue is a soft glue (i.e. a dissolvable glue) thenthe protrusions may be removed from the mask by dissolving the glue.This also removes the glue from the mask. If the glue is a hard glue(i.e. does not dissolve in solvents which are compatible with the mask)then the protrusions are mechanically removed from the mask. The hardglue is then mechanically removed from the mask. In alternativeembodiments, as mentioned above, other forms of bonding of theprotrusions are envisaged, such as magnetic or electrostatic attachment,optical bonding, or mechanical clamping. Where these are used theprotrusions are removed using an appropriate technique (e.g. whenelectrostatic attachment is used the voltage used to provide theattachment is removed).

Once the protrusions and the glue have been removed from the mask themask cleaning of the mask is performed. Replacement protrusions may thenbe glued onto the mask to receive a pellicle frame and pellicle.Inspection of the mask pattern for contamination may be performed beforeand/or after the protrusions have been glued to the mask. The pelliclemay be inspected for contamination before and/or after it has beenattached to the mask.

FIG. 8 depicts in more detail the process performed in the lithographicFAB. Parts of the process which are performed inside a lithographicapparatus are identified by a dashed line.

A mask assembly held in a container (which may correspond with thecontainer described above in connection with FIG. 5 ) is received at thelithographic apparatus. The container is put into a load-lock of thelithographic apparatus and is pumped down to a vacuum. The mask assemblyis then removed from the container. The back surface of the mask isinspected for contamination using an inspection tool located inside thelithographic apparatus. The pellicle is inspected for contaminationusing an inspection tool located in the lithographic apparatus. If nocontamination is found then the mask is used by the lithographicapparatus to project patterns onto substrates.

Once exposure of substrates has been completed the mask assembly is putback into the container in the load lock. Gas is introduced into thecontainer and the load lock, and the container and mask assembly areremoved from the lithographic apparatus.

If contamination is found then the mask assembly is removed from thelithographic apparatus. This involves placing the mask assembly backinto the container in the load lock, then introducing gas into thecontainer and the load lock. The mask assembly and container are thenremoved from the load lock. The next steps then depend upon the natureof the contamination that was found. If contamination was only found onthe pellicle, then the pellicle may be replaced with a new pellicle (andpellicle frame). This may be performed in a pellicle removal andattachment tool as described further above.

If contamination on the mask pattern is suspected or found to be present(or contamination on the back surface of the mask), then an inspectionof the mask using a mask inspection tool may be performed. This may befollowed by cleaning of the mask to remove the contamination. Cleaningof the mask may be performed with the protrusions remaining in place onthe mask. Following cleaning of the mask a further inspection of themask may then be performed using the mask inspection tool. If the maskis found to be free of contamination, then a pellicle and frame areattached to the mask and the mask is then transported in a container tothe lithographic apparatus.

If the cleaning has not removed the contamination, then the mask isreturned to the mask shop for further cleaning. This further cleaningmay be performed with the protrusions remaining in place. Alternatively,the protrusions may be removed before the further cleaning takes place.

A tool which removes and attaches pellicles to a mask may be separatefrom a pellicle inspection tool. Alternatively, a single tool may beprovided which removes and attaches pellicles to a mask and alsoinspects pellicles.

FIG. 9 schematically shows a method of making a pellicle assemblyaccording to an embodiment of the invention. Referring first to FIG. 9Aa pellicle is formed on a silicon wafer 50. Chemical vapour deposition(CVD) is used to deposit polysilicon onto the wafer 50. Capping materialmay be deposited on top of the polysilicon. A rectangular area of thesilicon wafer 50 is then etched away, leaving behind a thin layer ofpolysilicon supported by a silicon wafer perimeter. The thin layer ofpolysilicon, which may be referred to as a membrane, forms a pellicle52. The thin layer of polysilicon may for example have a thickness ofless than 100 nm, and may for example have a thickness of around 50 nm.In an embodiment, the pellicle may measure around 80 mm by 80 mm.

The presence of the silicon wafer 50 around the perimeter of thepellicle 52 is advantageous because it provides a rigid frame whichpreserves tautness of the pellicle 52. The pellicle 52 is taut when itis created due to the manner in which it is formed. The nature ofcrystallization of the polysilicon causes some shrinkage of thepolysilicon. This shrinkage removes wrinkles from the pellicle 52 andgives it tautness (which may be considered to be a pre-stressing of thepellicle). If the wafer 50 did not provide a rigid frame to support thepellicle 52, and instead a frame with flexibility was provided, then thetautness of the pellicle 52 would bend the frame inwards. As a result ofthis inward bending the tautness of the pellicle 52 would be lost. Theremaining steps of the method allow the outer portion of the wafer 50 tobe removed without tautness of the pellicle 52 being lost. It tautnessof the pellicle 52 were to be lost then uncontrolled sagging of thepellicle would occur and wrinkles would be seen in the pellicle.

A portion of the wafer 50 which extends around the outer edge of themembrane of the pellicle 52 may be referred to as a border portion 55 ofthe pellicle (an outer edge of the border portion is indicated by adashed line).

FIG. 9B schematically shows covers which are clamped to the wafer 50.The drawing on the left hand side of FIG. 9B shows a top side cover 54and the wafer 50 viewed from above. The drawing on the right hand sideof FIG. 9B shows the top side cover 54, wafer 50 and other componentsviewed in cross-section. The top side cover 54 is pressed against theborder portion 55 on the side of the pellicle which will be furthestfrom a mask in use.

The dashed line on the right hand side of FIG. 9B indicates the locationof the membrane of the pellicle 52. In this embodiment the membrane ofthe pellicle 52 at the bottom side of the wafer 50. This is because theetch that was used to remove the rectangular area of the wafer wasapplied to the top side of the wafer. In such an embodiment there is aclearance between the membrane of the pellicle 52 and the top side cover54. The top side cover 54 may therefore have a flat inner surface. In analternative embodiment the membrane of the pellicle 52 is at the topside of the wafer 50 (the etch was applied to the bottom side of thewafer). In such an embodiment there is no clearance between the membraneof the pellicle 52 and the top side cover 54, and the top side coverwill therefore include a recess to accommodate sagging of the pellicle.

A frame 58 and a bottom side cover 56 are provided on the opposite sideof the wafer 50. The frame 58 is fixed to the border portion 55. Theframe 58 is sufficiently rigid that it is capable of resisting inwardbending and thus can preserve tautness of the pellicle 52. The frame 58may be fixed to the border portion 55 using glue or any other suitablemeans. The bottom side cover 56 is pressed against the wafer 50 andcovers both the bottom side of the pellicle membrane 52 and the frame58.

From FIG. 9B it may be seen that the top side cover 54 covers thepellicle membrane 52 on the top side, and the bottom side cover 56covers the pellicle membrane on the bottom side. Thus, between them thecovers 54, 56 form a sealed enclosure which contains the pelliclemembrane 52. The top side cover 54 and the bottom side cover 56 andframe 58 are fitted to the wafer 50 in clean conditions in order tominimize the possibility of contamination being introduced into theenvironment of the pellicle membrane 52 when they are attached to thewafer 50. Indeed, the entire process of fabricating the pellicle andthen fitting the frame 58 and covers 54, 56 may be performed in cleanconditions.

As schematically depicted in FIG. 9C, a cutting tool (e.g. a millingmachine) is used to trim away parts of the wafer 50 which extend beyondthe bottom side cover 56. In FIG. 9C the right hand portion of the wafer50 has already been removed. The top portion of the wafer 50 is about tobe removed by cutting in the direction indicated by the arrow 60. Otherportions of the wafer 50 will then be removed. Because the pelliclemembrane 52 is contained within a sealed environment this cutting awayof the wafer does not risk introducing contamination onto the pelliclemembrane.

Once the edges of the wafer 50 have been trimmed away, the remainingassembly is a pellicle assembly 62 as shown in FIG. 9D. The pellicleassembly comprises a pellicle membrane 52, a substrate border portion55, a frame 58, a top side cover 54 and a bottom side cover 56. Thepellicle assembly 62 holds the pellicle membrane 52 in a sealedenvironment into which contamination cannot enter. The frame 58 supportsthe pellicle and maintains its tautness.

In the illustrated embodiment the bottom side cover 56 covers the frame58. This is advantageous in embodiments in which holes are provided inthe frame. Such holes are intended to allow the passage of gas duringuse of the pellicle, but contamination could pass through the holes tothe pellicle membrane 52 during other times. The bottom side cover 56prevents this from happening by providing a seal between the bottomcover and the substrate border portion 55 which isolates the holes fromthe external environment.

The top side cover 54 and bottom side cover 56 are pressed against thesubstrate border portion 55 by one or more clamps. The one or moreclamps may be of conventional construction.

The steps described and illustrated in connection with FIG. 9 provide apellicle assembly 62 which maintains pellicle tautness and preventspellicle contamination. The pellicle assembly 62 may, for example, bemanufactured at a single location. This is advantageous compared with,for example, manufacturing a pellicle at a first manufacturing locationand then transporting that pellicle to a second location to be fitted onto a supporting frame (contamination might be introduced duringtransportation to the second location).

The pellicle assembly 62 may, for example, be shipped from the pelliclemanufacturing location to a mask shop where the pellicle is fitted to amask for use by a lithographic apparatus. FIG. 10 shows schematically aprocess for attaching the pellicle to a mask. The process may, forexample, be performed at a mask shop (i.e. a factory where a patternedmask is created).

In a first step (not illustrated) the pellicle assembly 62 is cleaned inorder to remove contamination from the outside of the pellicle assembly.Following cleaning the pellicle assembly 62 is held in a cleanenvironment to avoid contamination being incident upon the pellicleassembly. Within the clean environment, a pellicle placement tool 64 isattached to the frame 58. The pellicle placement tool 64 includes arms66 which are received in blind bores provided in the frame 58 (i.e.openings provided on an outer surface of the frame which do not passfully through the frame). The pellicle placement tool 64 securely holdsthe frame 58 and presses the top side cover 54 against the substrateborder portion 55 (thereby holding the top side cover 54 in place). Oncethe pellicle placement tool has been attached to the frame 58 the one ormore clamps which are pressing the covers 54, 56 against the borderportion 55 are removed. The bottom side cover 56 can then be removedfrom the frame 58 as depicted in FIG. 10A. As depicted, the bottomsurface of the pellicle membrane 52 is thereby exposed. The frame 58 isalso exposed.

Referring to FIG. 10B, the pellicle placement tool 64 is used toposition the pellicle membrane 52 and frame 58 relative to a mask MA andto press the frame 58 on to the mask. The frame 58 may be secured to themask MA in any suitable manner. This may for example comprise attachingthe frame to attachment features which are provided on the mask MA (asdescribed further above). The pellicle placement tool 64 is thenremoved. This takes with it the top side cover 54.

The resulting assembly 70, which may be referred to as a mask assembly,is shown in FIG. 10C. The mask assembly 70 comprises a mask MA to whicha pellicle frame 58 and pellicle 52 are secured. The mask assembly 70may be stored and/or transported in a suitable container (which may, forexample, correspond with the mask assembly container described furtherabove).

Although FIGS. 9 and 10 describe the embodiment of the invention interms of a silicon wafer 50, other suitable substrates may be used.

FIG. 11 schematically depicts a method of monitoring a pellicleaccording to an embodiment of the invention. The method begins with amask assembly which is being used in a lithographic apparatus to exposesubstrates.

A first step of the method is in situ measurement of a property of thepellicle (or more than one property of the pellicle). Referring to FIG.1 , in situ measurement of a property of the pellicle means ameasurement which is performed when the mask assembly 15 is being heldby the support structure MT. If a change of the property is seen whichis associated with an increased risk of pellicle breakage, then thepellicle assembly is removed from the mask and replaced with a newpellicle assembly.

In an embodiment, an situ measurement of the pellicle may be performedusing an infra-red sensor. During exposure of a substrate the pellicleis heated by EUV radiation which is absorbed by the pellicle.Consequently, the pellicle will emit infra-red radiation, with thewavelength of that radiation being linked to the temperature of the ofthe pellicle. If the wavelength of the infra-red radiation shifts to ashorter wavelength, then this indicates that the temperature of thepellicle has increased. A significant temperature increase of thepellicle may indicate damage to the pellicle which increases the risk ofthe pellicle breaking. Therefore, the mask assembly is removed from thelithographic apparatus, and the pellicle assembly replaced with a newpellicle assembly.

In an embodiment, deformation of the pellicle which occurs duringscanning movement of the mask assembly may be measured. The deformationmay for example be a deflection of the pellicle towards the mask, andmay for example be determined by using a lateral shear interferometer tomeasure wavefront aberrations in EUV radiation which has passed throughthe pellicle. An increase or decrease of the deformation, for examplecompared with deformation observed when the mask assembly was firstsubjected to scanning movement, indicates a change of the stress of thepellicle. If the increase or decrease of the stress of the pelliclecorresponds with an increased risk of pellicle breakage then the maskassembly is removed from the lithographic apparatus and the pellicleassembly is replaced with a new pellicle assembly.

The next step of the method is determining whether a predeterminedperiod has elapsed since the last off-line inspection. The term“off-line” may be interpreted as meaning an inspection which takes placewhen the pellicle assembly is not in situ in the lithographic apparatus(i.e. when the mask assembly is not being held by the supportstructure). The predetermined period may be based upon the statisticallikelihood of the pellicle becoming damaged as a function of time.

When the predetermined period has elapsed the mask assembly istransferred to a mask assembly inspection tool. This may compriseremoving the mask assembly from the lithographic apparatus. The maskassembly may be placed in a container (e.g. corresponding with thecontainer 30 depicted in FIG. 5 ) for transfer from the lithographicapparatus to the mask assembly inspection tool.

The mask assembly inspection tool inspects the mask assembly to monitorfor damage of the pellicle. The mask assembly inspection tool measures aproperty (or more than one property) of the pellicle. If a change of theproperty is seen which is associated with an increased risk of pelliclebreakage during operation of the lithographic apparatus (e.g. thepellicle is found to be damaged), then the pellicle assembly is removedfrom the mask assembly and replaced with a new pellicle assembly.

If the pellicle is not found to be damaged by the mask assemblyinspection tool, then the pellicle assembly (i.e. pellicle and pellicleframe) may be detached from the mask using a pellicle frame detachmenttool (e.g. as described further above). Following this detachment thepellicle assembly is handled separately from the mask. The pellicleassembly is transferred to a pellicle inspection tool. The pellicleassembly may be located in a sealed container during this transfer. Themask is transferred to a mask inspection tool. The mask may be held in asealed container during this transfer. Inspection of the pellicleassembly by the pellicle inspection tool may be performed in parallelwith inspection of the mask by the mask inspection tool.

The pellicle inspection tool measures a property (or more than oneproperty) of the pellicle. If a change of the property is seen which isassociated with an increased risk of pellicle breakage during operationof the lithographic apparatus (e.g. the pellicle is found to bedamaged), then the pellicle assembly is removed from the mask assemblyand replaced with a new pellicle assembly. For example, if the pellicleis found to be damaged then the pellicle assembly may be replaced with anew pellicle assembly.

If the mask is found to be contaminated then the mask is cleaned inorder to remove the contamination. If cleaning of the mask does notremove the contamination then the mask is replaced with a new mask.

Once the pellicle has been confirmed as being undamaged (or has beenreplaced with a new pellicle), the mask and the pellicle assembly aretransported to the mounting/demounting tool (e.g. using sealedcontainers) where the pellicle assembly is mounted on the mask. Thepellicle assembly is then transported back to the lithographic apparatus(e.g. in a sealed container). Exposure of substrates using the maskassembly can then be performed by the lithographic apparatus.

Embodiments of the invention monitor for damage of the pellicle whichcauses an increased risk of pellicle breakage (which may be referred toas pellicle failure) during subsequent operation of the lithographicapparatus. When such damage is found the pellicle assembly is removedand replaced with a different pellicle assembly. This is advantageousbecause it minimizes the risk of a pellicle failing during operation ofthe lithographic apparatus. Pellicle failure during operation of thelithographic apparatus is undesirable because it could causecontamination of the mask and/or the lithographic apparatus.

In an embodiment (not depicted in FIG. 11 ), inspection of the maskassembly (e.g. inspection of the pellicle in situ on the mask) may beperformed more frequently than separate inspection of the pellicleassembly. Where this is the case, if the inspection of the mask assemblydoes not find pellicle damage associated with an increased risk ofpellicle breakage, then the mask assembly may be returned to thelithographic apparatus LA without separation of the mask and pellicleassembly and additional inspection thereof.

The mask assembly inspection tool may use one or more of the followingmeasurement techniques to measure one or more properties of thepellicle: EUV reflection measurements, EUV transmission measurements,ellipsometry, Raman spectroscopy, X-ray reflection measurements,microscope inspection, resonance measurements, scanning heat loadmeasurements, pellicle deflection during pumpdown or venting. These areeach described below:

EUV reflection measurements—EUV radiation is directed onto the pellicleand a sensor monitors for localized variations in the reflection of thepellicle. A localized variation in EUV reflection is indicative of adeterioration (or other change) of capping material on the pellicle.This deterioration or change of the capping material is indicative of abreakage risk of the pellicle. If such a deterioration or other changeis found then the pellicle assembly is removed from the mask and isreplaced. EUV reflection measurements may also monitor for globalvariations in the reflection of the pellicle. Again, a variation in EUVreflection (compared with a reference value of reflection, which may bea previously measured value) is indicative of a deterioration or otherchange of the capping material on the pellicle. Again, if such avariation is seen then the pellicle assembly is removed from the maskand replaced.

EUV transmission measurements (pellicle in situ on mask)—an EUVradiation beam is directed onto the pellicle. EUV radiation which passesthrough the pellicle is reflected by the mask and passes back outthrough the pellicle. This reflected EUV radiation is monitored. Themonitoring may be done by measuring and mapping the EUV radiation beforethe mask assembly is used, and then comparing a subsequently measuredmap with the initial map. Differences between the maps indicate either achange in the pellicle or a change in the mask. The nature of thedifferences may be used to discriminate a change of the pellicle from achange of the mask. If a significant change of the pellicle is seen thenthe pellicle assembly may be replaced. If a significant change of themask is seen then the mask may be cleaned.

Ellipsometry—this technique measures changes in the reflection of thepellicle over a range of wavelengths. If the measured spectrum ofreflected radiation changes (e.g. compared with a previously performedreference measurement) then this is indicative of changes in materialproperties of the pellicle (e.g. oxidation). These changes may indicatean increased risk of pellicle breakage. In addition, the changes ofmaterial properties may impact the optical performance of the pellicleduring lithographic exposure. Thus, when changes in material propertiesof the pellicle are determined by ellipsometry then the pellicleassembly is replaced.

Raman spectroscopy—this technique measures local changes in the stressof the pellicle. Raman spectroscopy is a spectroscopic technique basedon inelastic scattering of monochromatic light. The monochromatic lightmay be provided from a laser source. When photons undergo inelasticscattering from the pellicle the frequency of those photons changes. Thechange in frequency of the photons depends upon stress in the pellicle.Therefore, changes of stress in the pellicle can be observed using Ramanspectroscopy. A change of stress in the pellicle may indicate anincreased risk of pellicle breakage. A change of stress in the pelliclemay be a global change or may be a localized change. A localized changeof stress may be referred to as a stress concentration. The pellicleassembly is replaced if a changes of stress in the pellicle is seen thatindicates an increased risk of pellicle breakage.

X-ray reflection measurements—this technique directs a beam of x-raysonto the pellicle at a grazing incidence angle, and measures theintensity of the specular reflection of the x-rays from the pellicle.The intensity of the reflected x-rays is analyzed to determine one ormore of the density, thickness or roughness of the pellicle. Theroughness may be surface roughness of the pellicle or roughness of aninterface between material layers of the pellicle. A significantdeviation of any of the density, thickness or roughness from valuesexpected in an undamaged pellicle may indicate an increased risk ofpellicle breakage. Where this is the case, the pellicle assembly isremoved and replaced with a new pellicle assembly.

Microscope inspection—a microscope may be used to inspect for localdefects in the pellicle. The inspection may be manual or may beautomated, e.g. using image analysis software to monitor for defects inthe pellicle. The inspection may determine the number and/or size and/orshape of particles and/or holes in the pellicle. If particles are foundor if a hole is found which gives rise to an increased risk of pelliclebreakage, then the pellicle assembly is removed and replaced with a newpellicle assembly. For example, if a hole is found in the pellicle thenthis may give rise to an unacceptable risk of pellicle breakage whenpumping the mask assembly down to a vacuum or venting the mask assembly(significant pressure differences on either side of the pellicle mayoccur during pumping or venting). Replacing the pellicle assemblyprevents such a breakage from occurring.

Resonance measurements—a vibration is applied to the pellicle assembly,the frequency of the vibration being adjusted until a resonant frequencyis found. This may be done before using the mask assembly when it isknown that the pellicle is not damaged. During subsequent inspection ofthe pellicle assembly a vibration is again applied to the pellicleassembly. A deviation of the resonant frequency from the previouslyobserved resonant frequency is indicative of a global change in thestress of the pellicle and/or a change in the stress in some other partof the mask assembly. If a change of resonant frequency is observedwhich is indicative of damage associated with an increased risk ofpellicle failure then the pellicle assembly is replaced.

Scanning heat load measurements—in this technique a source of heat suchas a laser beam is scanned over the pellicle. At the same time thetemperature of the pellicle is measured, for example using a pyrometer.The pyrometer may be used to identify localized hot areas on thepellicle (i.e. areas which are hotter than the rest of the pellicle). Ifa localized hot area, which may be referred to as a hotspot, is seenthen this may be indicative of an increased risk of failure of thepellicle. Where this is the case the pellicle assembly is removed andreplaced. The heat delivered to the pellicle will cause a wrinklingpattern to be generated on the pellicle. The period (or other features)of this wrinkling pattern is linked to the stress of the pellicle.Therefore, the wrinkling pattern may be analyzed to determine whetherthe stress of the pellicle is such that there is a an increased risk offailure of the pellicle. If an increased risk of failure is present thenthe pellicle assembly is removed and replaced.

Pellicle deflection during pumpdown or venting—the mask assembly may betransferred to a chamber which can be pumped down to a vacuum or can bevented to atmospheric pressure. The chamber may be initially atatmospheric pressure when it receives the mask assembly. The chamber isthen pumped down to a vacuum in a controlled manner. As explainedfurther above in connection with FIG. 2 , a gap exists between thepellicle frame and the mask but the gap is relatively narrow andrestricts the flow of gas. As a result, the when the chamber is beingpumped down to vacuum the pressure between the pellicle and mask will behigher than the pressure of the chamber. This pressure difference willcause outward deflection of the pellicle, which is measured using asuitable sensor (e.g. a camera). The chamber may subsequently be ventedto atmospheric pressure in a controlled manner. This will cause inwarddeflection of the pellicle, which again may be measured using a suitablesensor (e.g. a camera). The degree of deflection of the pellicle isdependent upon the stress of the pellicle. A deflection which fallsoutside of predetermined threshold values may indicate an increased riskof pellicle failure.

Inspection of the pellicle when the pellicle assembly has been removedfrom the mask may comprise one or more of the following methods: EUVtransmission measurements, EUV reflection measurements, birefringencemeasurements, ellipsometry, Fourier transform infra-red spectroscopy,Raman spectroscopy, X-ray reflection measurements, microscopeinspection, resonance measurements, measurement of pellicle displacementdue to pressure difference, pellicle deflection during pumpdown orventing, scanning heat load measurements, frame deformationmeasurements. The majority of these are as described above. Those whichhave not been described above, or which may take a different form whenthe pellicle assembly has been removed from the mask, are describedbelow:

EUV transmission measurements (pellicle assembly removed from mask)—anEUV radiation beam is directed onto the pellicle and the amount of EUVradiation which is transmitted by the pellicle is measured using asensor located on an opposite side of the pellicle. This allowslocalized changes in transmission of the pellicle to be measured. Forexample, a test criterion for a pellicle may be transmission of 85% plusor minus 2%. If the transmission of the pellicle is higher than this(e.g. 87% or more) then this may indicate that a loss or material (e.g.capping layer material) from the pellicle has occurred. In thissituation an increased risk of pellicle failure may arise, and thepellicle assembly may therefore be replaced with a new pellicleassembly. If the transmission of the pellicle is lower than the testcriterion (e.g. 83% or less) then this may indicate that oxidation ofthe pellicle (e.g. oxidation of the capping layer) has occurred. Anincreased risk of pellicle failure may arise from the oxidation, and thepellicle assembly may therefore be replaced with a new pellicleassembly.

Birefringence measurements—birefringence measurements, which may also bereferred to as photoelasticity measurements, may be used to measurelocalized changes in the stress of the pellicle film. Birefringence mayfor example is measured by directing a radiation beam through thepellicle and measuring changes of the polarization of the radiationbeam. Measurements of the birefringence of the pellicle may be used tofind changes in the stress of the pellicle and/or localized stressconcentrations. When stress changes or localized stress concentrationsare seen which indicate an increased risk of pellicle failure, thepellicle assembly may be replaced with a new pellicle assembly.

Fourier transform infra-red spectroscopy—infra-red radiation (e.g. overa range of wavelengths) is directed towards the pellicle and theabsorption of that infra-red radiation is measured. This may be used tomonitor for localized changes of infra-red absorption of the pelliclefilm. The technique can be used to monitor for localized changes of theemissivity of the pellicle. For example, a minimum emissivity value forthe pellicle may be set as 0.3. If the emissivity (e.g. localizedemissivity) is lower than 0.3 then this may indicate damage of thepellicle. The lower emissivity could cause a localized temperatureincrease of the pellicle during use in the lithographic apparatus whichin turn gives rise to an increased risk of pellicle breakage. Thepellicle assembly is therefore replaced with a new pellicle assembly.

Measurement of pellicle displacement due to pressure difference—thisinvolves applying a pressure on one side of the pellicle which isdifferent to the pressure on the other side of the pellicle. Thepellicle will deflect towards the lower pressure side. The degree ofdeflection is dependent upon the stress of the pellicle, and adeflection which falls outside of predetermined threshold values mayindicate an increased risk of pellicle failure. In one example, amaximum threshold deflection of 500 μm for a pressure difference of 2Pascals may be set. If the deflection is larger than 500 μm then thisindicates a significant risk of pellicle breakage (e.g. during pumpdownor venting), and the pellicle assembly is therefore replaced with a newpellicle assembly. In another example, if the deflection is less than400 μm then this may indicate that the stress in the pellicle issignificantly higher than the stress in the pellicle as originallyfabricated (i.e. as attached to the pellicle frame but before use in thelithographic apparatus). A significant increase of the stress in thepellicle may mean an increased risk of pellicle breakage during use bythe lithographic apparatus. The pellicle assembly is therefore replacedwith a new pellicle assembly.

Frame deformation measurements—this involves applying force to thepellicle frame to cause a deformation of the pellicle frame, and thenmonitoring wrinkles of the pellicle which occur during the pellicleframe deformation. The positions of wrinkles in the pellicle areindicative of the stress in the pellicle. An initial measurement of thepositions of the wrinkles may be performed before the pellicle is usedin order to provide a reference measurement. Following use, a change ofposition of the wrinkles compared with that seen in the referencemeasurement indicates a change in the stress of the pellicle. If asignificant change of the stress of the pellicle is seen which isassociated with an increased risk of pellicle breakage, then thepellicle assembly is replaced with a new pellicle assembly.

As mentioned further above, inspection of the pellicle after removalfrom the mask may be performed in parallel with inspection and/orcleaning of the mask.

Monitoring the pellicle, for example using one or more of the abovetechniques, allows damage of the pellicle to be identified early, andtherefore allows the pellicle assembly to be replaced before failure ofthe pellicle occurs. If failure of the pellicle were to occur in thelithographic apparatus, e.g. during exposure of a substrate, then thiscould cause problematic contamination of the lithographic apparatus.This issue is avoided by monitoring for damage of the pellicle which isassociated with an increased risk of pellicle failure, and replacing thepellicle as necessary when such damage is found.

Inspection of the pellicle for contamination may be performed at thesame time as inspecting for pellicle damage.

Embodiments of the invention which relate to monitoring for damage ofthe pellicle may be combined with other embodiments of the inventiondescribed elsewhere in this document.

Various inventive aspects of a mask assembly have been described aboveand are shown in the figures in the context of specific embodiments ofthe invention. Various aspects of various methods have been describedabove. It will be appreciated that any of the described and/orillustrated aspects may be combined in a single embodiment. For example,one or more features of one embodiment may be combined with one or morefeatures of another embodiment. It will further be appreciated thatwhilst some embodiments have been described that include more than oneinventive aspect, embodiments that comprise only a single inventiveaspect are also contemplated herein. In general any of the features ofany of the described embodiments may be used in isolation or may be usedin any combination with any of the other features of the describedembodiments.

Although specific reference may be made in this text to embodiments ofthe invention in the context of a lithographic apparatus, embodiments ofthe invention may be used in other apparatus. Embodiments of theinvention may form part of a mask inspection apparatus, a metrologyapparatus, or any apparatus that measures or processes an object such asa wafer (or other substrate) or mask (or other mask). These apparatusmay be generally referred to as lithographic tools. Such a lithographictool may use vacuum conditions or ambient (non-vacuum) conditions.

The term “EUV radiation” may be considered to encompass electromagneticradiation having a wavelength within the range of 4-20 nm, for examplewithin the range of 13-14 nm. EUV radiation may have a wavelength ofless than 10 nm, for example within the range of 4-10 nm such as 6.7 nmor 6.8 nm.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications. Possible other applications include the manufactureof integrated optical systems, guidance and detection patterns formagnetic domain memories, flat-panel displays, liquid-crystal displays(LCDs), thin film magnetic heads, etc.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The descriptions above are intended to beillustrative, not limiting. Thus it will be apparent to one skilled inthe art that modifications may be made to the invention as describedwithout departing from the scope of the claims and clauses set outbelow.

1. A method comprising the steps of:

receiving a mask assembly comprising a mask and a removable EUVtransparent pellicle held by a pellicle frame;

removing the pellicle frame and EUV transparent pellicle from the mask;

using an inspection tool to inspect the mask pattern on the mask; andsubsequently attaching to the mask an EUV transparent pellicle held by apellicle frame.

2. The method of clause 1 further comprising:

after removing the pellicle frame and EUV transparent pellicle from themask, attaching to the mask an alternative pellicle frame holding analternative pellicle formed from a material which is substantiallytransparent to an inspection beam of the inspection tool; and

after using an inspection tool to inspect the mask pattern on the mask,removing the alternative pellicle held by the alternative pellicle framefrom the mask in order to attach to the mask the EUV transparentpellicle held by the pellicle frame.

3. The method of clause 1 or 2, wherein removing the pellicle frame fromthe mask comprises disengaging an attachment mechanism from anattachment feature, and attaching the pellicle frame to the maskcomprises engaging the attachment mechanism to an attachment feature.4. The method of clause 3, wherein the attachment feature is coupled tothe mask and wherein the attachment mechanism is coupled to the pellicleframe.5. The method of clauses 3 or 4 wherein the attachment feature iscoupled to the mask also after removing the pellicle frame and EUVtransparent pellicle from the mask by disengaging the attachmentmechanism from the attachment feature, such that the attachment featureis available for subsequent attachment of an EUV transparent pellicleheld by a pellicle frame after inspecting the mask pattern on the mask.6. The method of any one of clauses 3 to 5, wherein the attachmentmechanism comprises a locking member configured to engage with anattachment feature comprising a protrusion.7. The method of any preceding clause, wherein the EUV transparentpellicle and pellicle frame which is subsequently attached to the maskis the same EUV transparent pellicle and pellicle frame that was removedfrom the mask.8. The method of any preceding clause, wherein the alternative pellicleis substantially transparent to a non-EUV radiation beam used by themask inspection tool.9. The method of clause 8, wherein the non-EUV radiation beam used bythe mask inspection tool is a DUV radiation beam.10. The method of any of clauses 2 to 7, wherein the alternativepellicle is substantially transparent to a particle beam used by themask inspection tool.11. The method of clause 10, wherein the particle beam used by the maskinspection tool is an electron beam.12. The method of any one of clauses 2 to 11, wherein the alternativepellicle is attached to the mask using an attachment mechanism which isused solely for the alternative pellicle and is not used for theattachment of the EUV transparent pellicle.13. The method of clause 12 wherein the alternative pellicle is attachedto the mask such that the attachment feature of the EUV transparentpellicle does not touch the alternative pellicle.14. The method of any preceding clause, wherein the mask is in a cleanenvironment throughout the method.15. The method of any preceding clause, wherein the method furthercomprises transferring the mask assembly inside a sealed container froma lithographic apparatus to a pellicle removal and attachment tool.16. The method of any preceding clause, wherein the method furthercomprises transferring one or more selected from the mask, the pellicleassembly or the mask assembly inside a sealed container from a pellicleremoval and attachment tool to a mask inspection tool.17. The method according to any one of clauses 1 to 14, wherein the maskinspection tool is integrated with the pellicle removal and attachmenttool such that the mask assembly stays in the same environment.18. The method of any preceding clause, wherein the method furthercomprises cleaning the mask or the pellicle.19. The method of clause 18 wherein the attachment feature remainscoupled to the mask during cleaning.20. The method of clause 18 wherein the attachment feature is removedfrom the mask before cleaning.21. The method of clause 15 or clause 16, wherein the sealed containerhas a recessed portion configured to accommodate sagging of thepellicle.22. The method of clause 21, wherein a separation between the recessedportion of the container and a plane of the pellicle of the maskassembly is between is between 0.5 mm and 2 mm.23. The method of clause 22, wherein a separation between the recessedportion of the container and a plane of the pellicle of the maskassembly is between is between 0.5 mm and 1 mm.24. A method comprising the steps of:

receiving a mask assembly comprising a mask and an EUV transparentpellicle held by a pellicle frame arranged to be removably attachable tothe mask;

removing the pellicle frame and EUV transparent pellicle from the mask;

attaching to the mask an alternative pellicle held by an alternativepellicle frame arranged to be removably attachable to the mask, whereinthe alternative pellicle is formed from a material different to thematerial used to form the EUV transparent pellicle, which material issubstantially transparent to an inspection beam of an inspection tool;

using the inspection beam in the inspection tool to inspect the maskpattern on the mask;

removing the alternative pellicle from the mask; and

subsequently attaching to the mask an EUV transparent pellicle held by apellicle frame.

25. A method according to clause 24 wherein the alternative pellicleframe is attached to the mask at a different location than the EUVtransparent pellicle frame.

26. A mask assembly container comprising an opening through which a maskassembly may be placed inside the container, and a seal which seals shutthe opening when the mask assembly is located inside the container,wherein the container has a floor configured to accommodate outwardsagging of the pellicle.27. The mask assembly container of clause 26, wherein the floor isbetween 0.5 mm and1 mm or more away from a pellicle plane when the mask assembly is heldin the sealed container.28. A mask provided with a protrusion configured to receive a pellicleframe attachment mechanism, wherein a bottom surface of the protrusionhas a lip which defines a recess in the surface of the base, and whereinthe protrusion is attached to the mask by glue in the recess.29. The mask of clause 28, wherein the volume of the glue is less thanthe volume of the recess.30. The mask of clause 28 or clause 29, wherein the glue pulls theprotrusion towards the mask such that the recess and the mask form asubstantially enclosed space which retains the glue.31. The mask according to clause 28 or 29 wherein the protrusioncomprises an opening in the lip such that the recess and the mask form aspace which is partially open for glue outgassing.32. The mask according to any of clauses 28 to 31, wherein theprotrusion is attached to substrate material of the mask.33. A pellicle assembly container comprising an opening through which apellicle assembly may be placed inside the container, and a seal whichseals shut the opening when the pellicle assembly is located inside thecontainer, wherein the container has a floor configured to accommodateoutward sagging of the pellicle.34. A mask provided with at least three protrusions configured toreceive a pellicle frame attachment mechanism, wherein the protrusionsare removably attached to the mask.35. The mask of clause 33, wherein the protrusions are attached tosubstrate material of the mask.36. A method of making a pellicle assembly, the method comprising:

forming a membrane on a substrate and etching away substrate material toexpose the membrane and thereby provide a pellicle membrane supported bya substrate perimeter;

attaching a support frame to a portion of the substrate which bordersthe membrane;

providing a first cover on one side of the substrate and a second coveron an opposite side of the substrate and clamping them together to forma sealed environment which contains the pellicle membrane.

37. The method of clause 36, wherein the first cover is clamped againstthe substrate.

38. The method of clause 36 or clause 37, wherein the second cover isclamped against the substrate.

39. The method of any of clauses 36 to 38, wherein the method furthercomprises cutting away parts of the substrate which project beyond thefirst and second covers.

40. The method of any of clauses 36 to 39, wherein the substrate is asilicon wafer.

41. The method of any of clauses 36 to 40, wherein the second covercovers the support frame such that the support frame is located withinthe sealed environment.

42. The method of any of clauses 36 to 41, wherein the first coverincludes a recess configured to accommodate sagging of the pelliclemembrane.

43. The method of any of clauses 36 to 42, wherein the method isperformed at a pellicle manufacturing location.

44. A method comprising the method of any of clauses 36 to 43, andfurther comprising forming a mask assembly by:

attaching a pellicle location tool to the support frame;

removing the second cover from the pellicle assembly;

attaching the support frame to a mask; and

removing the first cover from the pellicle assembly using the pelliclelocation tool.

45. The method of clause 44, wherein the pellicle location tool includesarms which are received in blind holes provided in the support frame.

46. The method of clause 44 or clause 45, wherein that method isperformed at a mask shop.

47. The method of any of clauses 44 to 46, wherein the method furthercomprises putting the mask assembly inside a container and sealing thatcontainer.

48. A pellicle assembly comprising a pellicle membrane extending from asubstrate border portion, a support frame attached to the substrateborder portion, a first cover and a second cover, wherein the first andsecond covers are provided on opposite sides of the substrate borderportion and form a sealed environment which contains the pelliclemembrane.49. The pellicle assembly of clause 48, wherein the second cover coversthe support frame such that the support frame is located within thesealed environment.50. The pellicle assembly of clause 48 or clause 49, wherein the firstand second covers are clamped against the substrate border portion.51. A mask provided with a protrusion configured to receive a pellicleframe attachment mechanism, wherein a base surface of the protrusion hasa groove such that a glue is drawn by capillary action in a volumeenclosed by the groove and the mask, such that the protrusion isattached by the glue to the mask and the groove is partially open forglue outgassing.52. A method of monitoring a pellicle of a mask assembly, the maskassembly comprising a pellicle assembly and a mask, the methodcomprising:

measuring a property of the pellicle and monitoring for a change of theproperty which is associated with an increased risk of pelliclebreakage, and

when such a change is seen removing the pellicle assembly from the maskand replacing it with a new pellicle assembly.

53. The method of clause 52, wherein the property of the pellicle ismeasured when the mask assembly is in situ in the lithographicapparatus.

54. The method of clause 53, wherein the property is infra-red emissionof the pellicle and/or is deflection of the pellicle during scanningmovement of the mask assembly.

55. The method of any of clauses 52 to 54, wherein the method comprisestransferring the mask assembly to a mask assembly inspection tool andthen measuring the property of the pellicle using the mask assemblyinspection tool.

56. The method of clause 55, wherein one or more of the followingmeasurement techniques are used to measure one or more properties of thepellicle: EUV reflection measurements, EUV transmission measurements,ellipsometry, Raman spectroscopy, X-ray reflection measurements,microscope inspection, resonance measurements, scanning heat loadmeasurements, pellicle deflection during pumpdown or venting.57. The method of any of clauses 52 to 56, wherein the method comprisesremoving the pellicle assembly from the mask, transferring the pellicleassembly to a pellicle assembly inspection tool and then measuring theproperty of the pellicle using the pellicle assembly inspection tool.58. The method of clause 57, wherein one or more of the followingmeasurement techniques are used to measure one or more properties of thepellicle: EUV transmission measurements (pellicle assembly removed frommask), EUV reflection measurements, birefringence measurements,ellipsometry, Fourier transform infra-red spectroscopy, Ramanspectroscopy, X-ray reflection measurements, microscope inspection,resonance measurements, measurement of pellicle displacement due topressure difference, deflection during pumpdown or venting, scanningheat load measurements, frame deformation measurements.

The invention claimed is:
 1. A pellicle inspection tool for an EUVtransparent pellicle, the pellicle inspection tool comprising: a supportstructure for supporting: a) a pellicle comprising the EUV transparentpellicle membrane supported by a substrate perimeter that borders themembrane; or b) a pellicle assembly comprising the EUV transparentpellicle membrane and a pellicle frame attached to the substrateperimeter that borders the membrane; or c) a mask assembly comprisingthe EUV transparent pellicle membrane, the pellicle frame attached tothe substrate perimeter that borders the membrane, and a mask attachedto the pellicle frame; and a device for measuring a property of the EUVtransparent pellicle and monitoring for a change of the property that isassociated with an increased risk of pellicle breakage, wherein the EUVtransparent pellicle is configured to transmit at least 65% of incidentEUV radiation.
 2. The pellicle inspection tool of claim 1, wherein thedevice is arranged to measure a property of the pellicle using one ormore measuring techniques selected from a group consisting of EUVreflection measurements, EUV transmission measurements, birefringencemeasurements, ellipsometry, Fourier transform infra-red spectroscopy,Raman spectroscopy, X-ray reflection measurements, microscopeinspection, resonance measurements, scanning heat load measurements,measurement of pellicle displacement due to pressure difference, andpellicle deflection during pumpdown or venting.
 3. The pellicleinspection tool of claim 1, wherein the device is an infra-red sensorarranged to (i) measure an infra-red radiation emitted by the pelliclewith the wavelength of that radiation being linked to the temperature ofthe of the pellicle, or (ii) monitor for changes of infra-red absorptionof the pellicle film in order to detect localized changes of theemissivity of the pellicle.
 4. The pellicle inspection tool of claim 1,wherein the device is a sensor arranged to monitor global or localizedvariations in the EUV reflection of the pellicle.
 5. The pellicleinspection tool of claim 1, wherein the device is a sensor arranged todetermine an amount of EUV radiation that is transmitted by thepellicle.
 6. The pellicle inspection tool of claim 1, wherein the deviceis arranged to measure changes in a reflection of the pellicle over arange of wavelengths, wherein the measured changes are indicative ofchanges in material properties of the pellicle.
 7. The pellicleinspection tool of claim 1, wherein the device is arranged to measurechanges in a polarization of a radiation beam to determine global orlocal changes in the stress of the pellicle.
 8. The pellicle inspectiontool of claim 1, wherein the device is arranged to measure an intensityof a specular reflection of X-rays from the pellicle.
 9. The pellicleinspection tool of claim 8, further comprising a source of X-ray beamsarranged to direct a beam of X-ray onto the pellicle at a grazingincidence angle.
 10. The pellicle inspection tool of claim 1, whereinthe device is arranged to measure a variation in a resonant frequency ofthe pellicle assembly or the mask assembly.
 11. The pellicle inspectiontool of claim 1, wherein the device is a sensor arranged to measure adegree of deflection of the pellicle due to a pressure difference. 12.The pellicle inspection tool according to claim 1, wherein the device isarranged to measure a position of wrinkles in the pellicle and/or achange in the position of the wrinkles.
 13. A lithographic apparatuscomprising a pellicle inspection tool for an EUV transparent pellicle,the pellicle inspection tool comprising: a support structure forsupporting: a) a pellicle comprising an EUV transparent pelliclemembrane supported by a substrate perimeter that borders the membrane;or b) a pellicle assembly comprising the EUV transparent pelliclemembrane and a pellicle frame attached to the substrate perimeter thatborders the membrane; or c) a mask assembly comprising the EUVtransparent pellicle membrane, the pellicle frame attached to thesubstrate perimeter that borders the membrane, and a mask attached tothe pellicle frame; and a device for measuring a property of the EUVtransparent pellicle and monitoring for a change of the property that isassociated with an increased risk of pellicle breakage, wherein the EUVtransparent pellicle is configured to transmit at least 65% of incidentEUV radiation.